Method for treating material samples and apparatus for testing exposure to light and weathering

ABSTRACT

The invention concerns the ageing of material samples in the test enclosuref an apparatus for testing exposure to light and weathering, using visible light, UV light and medium- or long-wavelength IR radiation and with air circulation. In order to ensure reproducible test results, a value representing the surface temperature of the sample is measured by means of a temperature sensor and compared, at the input of a control unit, with a predefined reference temperature. In the event of a temperature difference, a control signal generated by the control unit adjusts the power supply to a separate IR radiant heater for medium- and long-wavelength IR radiation in such a way that the difference disappears in accordance with a predefined indicial response.

The invention relates to methods for regulating the temperature ofmaterial samples in a light and weathering test apparatus with aclosed-off chamber for samples, in which by means of a radiationarrangement visible light, UV and IR radiation is generated, and to alight and weathering test apparatus.

From German Patent DE 20 14 288, a light and weathering fastness testingapparatus with a sample chamber is known, in which samples located on acylinder jacket surround a radiation source, and between this source andthe samples a cylindrical mirror that selectively reflects the infraredcomponent of the radiation and is permeable to the visible and UVcomponents is provided; the effect of radiation in the short-waveinfrared range from 800 to 1000 nm is considered especially problematic.Located in the middle of the cylindrical mirror is a metal tube throughwhich a coolant flows, so that the intensity of the infrared radiation,simultaneously produced and occurring in the form of heat, whichintensity grows with the intensity of the UV radiation can bedissipated; in this way, influence on the samples by breakdown reactionsresulting from infrared radiation are largely avoided, so that anychanges in the sample can be ascribed solely to the effect of UVradiation.

From European Patent Disclosure EP 0 320 209 A1, a weathering testsystem is also known in which the inside surfaces of samples are locatedon the inside of an approximately spherically formed mounting stand andexposed to an airstream, passed through it by a blower, and to a lightsource; with the aid of a regulating device for the blower, apredetermined temperature is maintained in the interior of the samplestand.

Moreover, from German Patent Disclosure DT 24 23 052 A1, a method and anapparatus for testing the weather resistance of samples is known inwhich for speeding up the testing of the samples, an artificial opticalirradiation from one or more radiation sources, which also outputultraviolet or superactinic radiation is provided during twilight,darkness and nighttime; the test stand provided for testing weatherresistance of samples in the open air has a sample carrier or aplurality of carriers for the radiation device with a guide and drivedevice; the radiation sources also output ultraviolet or superactinicradiation, and during the artificial irradiation at night no abnormallyelevated temperatures. are attained, since high sample temperatures cancause a different kind of change to the bodies of the samples from thatcaused in natural weathering in the open air.

It proves to be a problem in the prior art that exact definition of thesurface temperature, which should be kept as constant as possible if thesamples are to be irradiated in a manner approximating actuality, cannotbe performed here. In particular it is not possible for the blackstandard temperature, measured by a black standard thermometer, to beadjusted independently of other parameters and hence it is not possibleto characterize temperature conditions of dark-colored sample bodieswith low heat conductivity in the plane of the sample mounts duringirradiation in accordance with DIN Standard 53387.

The object of the invention is thus to disclose a method for adjustingor keeping constant the surface temperature of the material samples andan apparatus for testing light and weather fastness having a samplechamber, in which the samples have an adjustable, optionally constantsurface temperature which is independent of the intensity of the UVradiation, and in which the sample chamber temperature and atmosphericair circulation or climate control fan speed are kept constant; by meansof filters, a radiation is to be output into the sample chamber thatapproximates global radiation, as disclosed for instance in thepublication of the Commission International de l'Eclairage InternationalLighting Commission! Publ. No. CIE85/1989, First Edition 1989, Table 4.

This object is attained in terms of the method by the characteristics ofthe body of claim 1.

In an advantageous feature of the method, the intensity of the IRradiation is regulated by comparison of a surface temperature measuredin the region of the sample with a predetermined set-point temperaturevalue, and the intensity of the IR radiation is controlled by a controlsignal of the regulator that depends on the difference or controldeviation, ascertained at the regulator input, of the sensor signal fromthe set-point value signal.

The relatively simple regulation of the surface temperature of thesamples as an individual parameter proves to be advantageous.

In terms of the apparatus, the object is attained by the characteristicsof the body of claim 3.

In an advantageous feature of the apparatus, the radiation arrangementhas at least one discharge lamp for emitting UV radiation and visiblelight as well as a separate temperature radiator as a source of infraredradiation; the intensity of the IR radiation output by the temperatureradiator is adjusted by triggering the current supply by means of acontrol signal of the regulator, or the control deviation is adjusted;by means of interference filters, IR radiation below a wavelength of1200 nm is absorbed or reflected, so that the IR radiation relevant tothe heating of the surfaces of the samples has a wavelength of more than1200 nm. An air stream with an approximately constant air throughputflows through the sample chamber by means of a blower.

Further advantageous features of the invention are disclosed in claims5-13.

One substantial advantage is considered to be the always-constantsurface temperature, because replicable test results can thus beattained; it also proves to be advantageous that a variation of thesurface temperature can be performed purposefully by regulating thetiming.

The subject of the invention will be described in further detail belowin conjunction with FIGS. 1, 2 and 3.

FIG. 1 schematically shows a cross section through the light andweathering test apparatus;

FIG. 2 shows the transmission of the filter characteristics and thespectral irradiation intensity of the infrared radiation source in thespectral range from 900 to 2500 nm;

FIG. 3 schematically, in the form of a block circuit diagram, shows theregulator for the light and weathering test apparatus.

As seen in a schematic cross-sectional view of the sample chamber inFIG. 1, the samples 1 are secured to a rotatable sample carrier 2, whichis set into constant rotation by means of a drive device, not shownhere. Located in the central region of the cylindrical housing 3 is aradiation arrangement comprising two discharge lamps 4, 5, forgenerating UV radiation and visible light, and one infrared radiator 7;the discharge lamps and the infrared radiator are closed off from thesample chamber 9 by a cylindrical filter 8, through which a coolantflows along the radiation passage faces and which in particular absorbsthe short-wave infrared components of the discharge lamps in the rangefrom 800 to 1000 nm, but at the same time is largely permeable toradiation in the long-wave ultraviolet range from 300 to 400 nm and inthe visible spectral range. The inside surface of the filter 8 is alsocovered from the radiators or discharge lamps with interference filters10 fitted against one another in the manner of a polygon, which arelargely permeable to the long-wave UV range from 300 to 400 nm and thevisible spectral range, while they largely absorb radiation in theinfrared wavelength range from 900 to approximately 1200 nm. From awavelength of approximately 1200 to 1250 on into the range of 2500 nm,the interference filters 10 are largely permeable to the medium- andlong-wave infrared radiation, with a transmission of 70 to 95%.

In the interior of the filter 8, a body 12 is provided which absorbs thereflected infrared radiation and through which a coolant, such as air orwater, flows for the sake of heat dissipation.

In the region of the rotatable sample carrier 2, a temperature sensor 13is located at the level of the surfaces of the samples 1; it measuresthe heat of radiation transmitted as a result of the infrared radiationof the IR radiator 7, or in other words measures the surface temperatureof the samples. With the aid of the regulating device described below,it is thus possible to adjust and regulate the black standardtemperature of the substances to be tested, on the basis of the samplechamber temperature of the temperature dependent on the medium flowingaround it, as one of the decisive parameters independently of otherparameters; the tolerances specified in DIN 75220 with regard to globalradiation, given in the aforementioned Table 4 of the CIE, should if atall possible be adhered to.

During normal operation it is possible to expose the samples to therelevant atmosphere for later use that flows through the sample chamber,such as air with a predetermined moisture content and a predeterminedflow speed.

The filter characteristic of the interference filters 10 of FIG. 1 willnow be described in detail, referring to curve A in FIG. 2. As can beseen from the wavelength plotted along the X axis, the transmissiongiven along the Y axis, in the range up to about 1130 nm, is only 5%,while in the range beyond 1250 nm it rises to a range of 70 or more than90%, so that the infrared spectrum, which impedes the process of testingthe samples, is kept away from them, while the relatively low-energyinfrared radiation of the medium- and longer-wave range reaches samplesand can be adjusted or regulated in the form of independent parameters.Heat reflection filters with a blocking range from about 800 to 1200 nm,which are available on the market, have proved especially suitable asinterference filters. It is also possible, however, to use other filterswith an appropriate transmission characteristic.

Characteristic curve B in the graph in FIG. 2 is oriented to thespectral radiation intensity in relative units of the infrared radiationsource 7, which is plotted along the Y' axis as a function of thewavelength given on the X axis; as can be seen from the course of curveB, the radiated intensity in the range from 900 to 1280 nm, is less than60 relative units of the spectral radiation intensity, while in therange from 1300 to 2500 nm it rises from a value of 60 to a value of 100pertaining to the relative radiation intensity, so that the spectralrange relevant to the black standard temperature can be transmitted witha high intensity to the plane of the samples.

The regulating device will be described in further detail in terms ofthe schematic illustration in FIG. 3; together with the temperaturesensor 13 and an analog/digital converter 16, the infrared radiator 7supplied by a controllable power supply 15 forms the controlled path 17of the regulating device. Via the input 19, by means of a control signalY, the power supply 15 is triggered in such a way that a predeterminedradiation intensity of the connected infrared radiator 7 can be attainedby means of regulation. A radiator with a tungsten incandescent coil,with a power takeup in the range from 500 W to 2 kW, has provenespecially suitable as the infrared radiator. The infrared radiationoutput by the radiator 7 to the sample chamber via infrared filters 10generates a heat in the plane of the samples whose temperature isascertained by the temperature sensor 13 and sent on in the form of anelectrical signal t to the analog/digital converter 16; this latterelement converts the analog signal t into a digital controlled variablex, which is delivered via the output 20 to a comparator 21 at the input23 of the regulator 22; while the controlled variable x is delivered forinstance as a negative digital signal, a predetermined digital set-pointvalue w from a set-point value specifying device 27 is delivered in theform of a positive digital value to the comparator 21, and the digitalcontrol deviation (w-x) is thus ascertained at the output of thecomparator and delivered to the regulator 22 via its input 23. Amicroprocessor 24 and a signal processor 25 are located inside theregulator 22, and as a result, a control signal y is generated at theoutput 26 of the regulator; it is used to control the power supply 15for the infrared radiator 7 via the input 19 and is effective as acontrol signal until such time as the control deviation becomes w -x=0,and thus the radiation 7 output by infrared radiators corresponds to thespecified set-point value w. Because of the digital design of theregulator 22, it is possible by programming the microprocessor as aclosed-loop control component to attain an optimal closed-loop controlbehavior.

If the surface temperature of the specimens is to be varied during thetreatment process, then it is possible instead of a fixedlypredetermined set-point value w to use a timing transducer with achronologically varied set-point value or with program regulation.

The use of an additional infrared radiator as a pure heat sourceaccording to the invention makes it possible for the surface temperatureof the samples, which otherwise depends on the absorption behavior ofmaterials to be tested, on the sample chamber temperature, and on thespeed of the medium flowing through, to be adjusted and regulated fullyindependently as one of the decisive parameters.

We claim:
 1. A method for regulating the temperature of material samplesin a light and weathering test apparatus with a closed chamber with aircirculation for material samples, in which chamber by means of aradiation arrangement, which includes at least one discharge lampembodied as a xenon radiator, visible light, UV and IR radiation aregenerated,characterized in that the IR radiation is generated by atemperature radiator; infrared radiation with a wavelength below 1200 nmis largely suppressed by means of filters; IR radiation with awavelength above 1200 nm is transmitted to the material samples; theheat produced on the surfaces of the sample by the IR radiation ismeasured by means of temperatures as a temperature signal and suppliedto a regulator for regulating the surface temperature to a predeterminedset-point temperature value by controlling the intensity of the IRradiation; and the radiation intensity of the temperature radiator isadjusted as a function of a control signal of the regulator, and thecontrol signal is dependent on the control deviation between theset-point temperature value and the measured temperature signal.
 2. Alight and weathering test apparatus with a closed-off chamber forsamples, in which a radiation arrangement surrounded by a rotatablesample holder is provided, which includes at least one discharge lampformed as a xenon radiator and emitting both UV radiation and visiblelight and which radiates visible light and UV and IR radiation to thesample,characterized in thatat least one temperature sensor (13) forregulating the intensity of the IR radiation is provided in the plane ofthe samples; and the radiation arrangement has at least one electrictemperature radiator (7) for emitting IR radiation, whose intensity isadjustable by means of triggering its current supply via the controloutput (26) of a regulator (22), and the input (23) of the regulator isconnected via a comparison element (21) to the temperature sensor (13)for the ascertaining the control deviation in the measured temperaturevalue from a predetermined set-point temperature value signal.
 3. Thelight and weathering test apparatus of claim 2, characterized in that atleast one filter (10) for adapting the radiation output by the lamp tothe global radiation is located between the sample chamber (9) and thedischarge lamp.
 4. The light and weathering test apparatus of claim 3,characterized in that the discharge lamp is surrounded, in the directionof the sample chamber (9), by a cylindrical filter (8) for absorbing theinfrared components output by the discharge lamp (4, 5).
 5. The lightand weathering test apparatus of claim 2, characterized in that at leastone filter (8, 10) for absorbing radiation with a wavelength below 1200nm is located between the temperature radiator (7) and the samplechamber (9).
 6. The light and weathering test apparatus of claim 5,characterized in that the temperature radiator, viewed in the directionof the sample chamber, is surrounded by at least one filter (8, 10) forabsorbing radiation with a wavelength below 1200 nm.
 7. The light andweathering test apparatus of claim 4, characterized in that thedischarge lamp (4, 5) and temperature radiator (7) are located insidethe cylinder filter (8).
 8. The light and weathering test apparatus ofclaim 7, characterized in that at least the temperature radiator (7) issurrounded by flat filters (10) arranged in the manner of a polygon, forabsorbing a wavelength below 1200 nm.
 9. The light and weathering testapparatus of claim 2, characterized in that the temperature sensor (13)located in the plane of the samples is formed as part of a closed-loopcontrol circuit which has a regulator (22) with comparison of theset-point and actual temperature values at the regulator input (23), andthe regulator output (26) is connected to a final control element (15)for controlling the radiation output of the temperature radiator (7).